An elastic wave device implemented using a 42 degree-rotated Y-cut lithium tantalate piezoelectric substrate has been commonly used for a filter and an antenna duplexer of a communication device. The elastic wave device is configured to use a plurality of comb-shaped electrodes (also referred to as interdigital transducer (IDT) electrodes) formed on the piezoelectric substrate. Recently, there has been a need for an elastic wave device having higher performance and better temperature characteristics. One approach to realizing such an elastic wave device involves making the piezoelectric substrate of lithium niobate and providing a silicon oxide film on the IDT electrode so that the temperature characteristic can be improved. This was expected to provide a higher Q at resonance, especially in an elastic wave device using lithium niobate that has a cut angle for the primary elastic wave to be a Rayleigh wave. However, spuriousness such as a spurious response in the transverse mode, and a spurious response due to an undesired wave other than the Rayleigh wave as the primary elastic wave can be caused by a plurality of factors in such an elastic wave device. This spuriousness can degrade the filtering characteristics of the elastic wave device.
FIG. 19 illustrates an example of the configuration of a conventional elastic wave resonator. In the example illustrated in FIG. 19, the elastic wave resonator has comb-shaped electrodes 1902 and reflector electrodes 1903 that are formed on a piezoelectric substrate 1901. This configuration experiences problematic spuriousness in the transverse mode, particularly when the piezoelectric substrate of the elastic wave resonator is made of lithium niobate.
A conventional approach to solving the spuriousness problem in elastic wave resonators such as that shown in FIG. 19 includes using apodization in the elastic wave resonator. For example, FIG. 20 illustrates a configuration of a conventional elastic wave resonator including apodization. In the example shown in FIG. 20, the elastic wave resonator has comb-shaped electrodes 2002 and reflector electrodes 2003 that are formed on a piezoelectric substrate 2001. Here, the comb-shaped electrode 2002 has apodization to suppress spuriousness in the transverse mode, as described, for example, in US patent application publication No. 2011/0068655.